In a die-casting process for fabricating a cast product having a complicated internal shape, a core is used. As a core used for such a die-casting process, there have been developed water-soluble cores, which are formed of salts, etc., and, after the casting, are removed from the cast products by dissolution with water (Patent documents 1-3).
In the case of the die-casting process using such a water-soluble core, especially in the case of performing the die-casting process at a high pressure, the velocity of the molten metal at the time of casting becomes large and also a high impact force is applied to the water-soluble core, so that a high strength is required of the water-soluble core. Accordingly, the reinforcement of the water-soluble core has been practiced but, if the water-soluble core is provided with an enhanced strength, the water-soluble core is liable to have inferior collapsibility required for removal thereof from the cast product.
Various methods have been proposed in order to promote collapse of water-soluble cores which are principally composed of soluble constituents, such as salts. For example, there has been proposed a method, wherein alkaline earth metal compound powder, such as quicklime (calcium oxide) or magnesium oxide, is admixed within an alkali metal salt as a main core component to form a water-soluble core, which is contacted with water after the casting of a metal to cause the expansion of the alkaline earth metal compound powder with water to promote the collapsibility of the water-soluble core (Patent document 4). However, according to a study of the present inventors, the addition of such an alkaline earth metal compound failed in a desired improvement in collapsibility but rather resulted in difficulties, such as inferior moldability (castability) of the core.